The present invention relates to weighing and delivery systems and more particularly to an improved method for controlling an amount of material delivered during a material transfer.
In many batch weighing/mixing systems, materials are fed into a mix tank in a sequence and according to amounts determined by a recipe, or formula. The materials are often fed into the mix tank at varying feed rates. In some systems, the feed rate varies from less than one pound per second to more than three hundred pounds per second. Regardless of the feed rates, it is desired to feed the exact amount of material into the batch in the shortest possible time. This means feeding at maximum feed rates for as long as possible and then, as the feed amount approaches a target amount, stopping the feed to achieve the exact desired amount of material without reducing the product feed rate.
During the feeding of the material, the scale reading at any moment is different from the actual amount of material fed or the final weight that would result if feed were stopped, or cut off, at that exact moment. The difference between the final batch weight and scale reading at cutoff is called Spill. For this reason, it is not possible to feed the material at maximum flow and then stop the feed when the scale reading reaches the target weight to achieve the exact desired amount of material.
One traditional approach to this problem uses a two-stage feed which switches to a slow, dribble feed as the scale reading reaches a set percentage (typically 80-90%) of the target. Variations on this approach use multi-stage feeds or throttling valves, but all have the effect of lengthening feed time. Accuracy can be improved by lowering the dribble rate, at the expense of feed time. Agitators in the process cause significant scale noise, which can only be reduced or eliminated mechanical or electronic filtering, increasing the tradeoff between accuracy and feed time.
What is needed is a method to quantify and predict what the Spill will be during the material feed. Furthermore, a method is needed to better control the amount of material delivered during a material transfer according to a desired amount.